Particle Size Distribution (PSD) testing is one of the most important assessments in bulk earthworks. Whether you’re building a subdivision, road pavement, structural platform or commercial foundation, PSD testing helps confirm whether soil or imported fill is suitable, stable and consistent for compaction.
Understanding how PSD works — and why it matters — helps contractors, builders and developers reduce risk, avoid compaction failures, and meet council or engineering specifications.
For an overview of the complete earthworks testing process in NSW, you can also refer to our guide here:
👉 https://idealgeotech.com.au/bulk-earthworks-testing-nsw/
This article breaks down the essentials of PSD testing earthworks, including the basics, its impact on ground performance, how testing is done, and when contractors should request it.
1. PSD Basics
PSD (Particle Size Distribution) describes how the particles within a soil or fill material are spread across different size ranges — from fine clays and silts to sands, gravels and coarse fragments.
What PSD Measures
PSD testing identifies:
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the percentage of each particle size present
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whether the material is well-graded or poorly graded
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whether the blend is appropriate for structural fill
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how the soil will behave under compaction and load
Well-Graded vs Poorly Graded Material
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Well-graded soils contain a balanced mix of particles and pack together tightly, improving compaction.
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Poorly graded soils (uniform sand or clay lumps, for example) contain particles in a narrow size range and often lead to voids or weak zones.
Why Engineers Care About PSD
Engineers use PSD to:
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classify soil
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compare fill against specifications
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assess suitability for compaction
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predict future performance under loads
It is one of the foundational tests used before large-scale fill placement.
2. How PSD Impacts Earthworks Performance
Soils behave differently based on their particle size makeup. PSD directly influences compaction quality, stability, and long-term structural performance of the platform or pavement being built.
2.1 Compaction Efficiency
Well-graded materials compact more densely and evenly, reducing the risk of:
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settlement
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pumping
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soft spots
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uneven ground
Poorly graded soils often fail density tests or require moisture adjustment before compaction.
2.2 Drainage and Moisture Behaviour
PSD affects how water moves through soil:
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Sandy or coarse soils allow rapid drainage
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Clay-heavy soils retain moisture and may shrink or swell
Knowing the PSD helps engineers predict seasonal or moisture-related changes in the subgrade.
2.3 Strength and Load-Bearing Capacity
PSD influences:
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shear strength
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supporting capacity under slabs
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rut resistance under traffic
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stability for retaining structures
High-quality fill for subdivisions or roads typically requires a PSD profile that meets project specifications.
2.4 Material Selection for Controlled Fill
Proper PSD ensures imported fill meets the criteria for controlled or engineered fill — a requirement for most subdivisions and commercial developments under Australian standards.
3. How PSD Testing Is Performed
PSD testing is carried out in a NATA-accredited geotechnical laboratory. The test separates soil particles into size groups and measures how much of each group is present.
3.1 Sample Collection
A representative soil or fill sample is taken from site or the stockpile. Proper sampling is important to avoid misleading results.
3.2 Sieve Analysis (Coarse Particles)
For sands and gravels:
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The material is dried
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Passed through a stack of sieves
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Each sieve captures a different particle size
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The retained weight on each sieve is recorded
This creates a detailed size distribution curve.
3.3 Hydrometer Analysis (Fine Particles)
For silts and clays, hydrometer testing is used to measure how fine particles settle in suspension, providing accurate PSD data for fine-grained soils.
3.4 Interpreting the PSD Curve
The PSD curve shows:
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grading behaviour
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compaction potential
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suitability compared to project specifications
Engineers review the curve to decide whether soil can be used, amended, or replaced.
4. When Contractors Should Request PSD Testing
You should consider PSD testing whenever:
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importing fill for earthworks
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building new subdivisions or structural pads
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constructing pavements or driveways
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working with unknown site-won materials
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fill consistency is unclear
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fill has failed density tests
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engineers request verification
PSD is especially important for any project requiring compliance with engineering drawings or council approvals.
5. Need PSD Testing for Your Earthworks Project?
If you want accurate, fast and NATA-accredited PSD testing for earthworks, we provide:
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PSD and Atterberg limits testing
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compaction testing and Level 1 supervision
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imported fill verification
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CBR and subgrade assessment
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AS 3798-compliant reporting
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council-ready documentation
Request PSD Testing for Your Project
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